The present invention relates to an article having a coating for reducing radiation heat transfer, in particular a reflective ceramic coating, and to a method for forming the coated article.
The application of thermal barrier coatings to turbine components is an effective method for increasing the working temperature of the turbine section and for improving overall engine efficiency. Thermal barrier coatings reduce the substrate temperatures of cooled articles, thereby increasing component service life while maintaining a given efficiency. They also maximize the effectiveness and efficiency of compressor exit air used to cool turbine components. Although surface temperatures of a turbine component may be higher than 2000° F., the surface temperature of the overlying ceramic thermal barrier coating will be as much as 300° F. hotter or more.
A typical state-of-the-art zirconium oxide-based thermal barrier coating 10 applied by electron beam physical vapor deposition to a nickel-based alloy substrate 12 is illustrated in FIG. 1. Prior to deposition of the coating, a metallic bond layer 14 is usually applied to the surface of the substrate. With electron beam physical vapor deposition processing, the oxide ceramic of the coating usually acquires a columnar morphology during growth. Yttrium, magnesium, calcium and/or other suitable oxide is typically added to the zirconium oxide to stabilize the tetragonal and/or cubic crystal structure required for coating durability.
The primary benefits of such zirconium oxide-based ceramic thermal barrier coatings are reduced metal temperatures and reduced cooling requirements. These benefits are derived from the inherently low thermal conductivity of the coating material. At higher-temperature, heat transport through a conventional ceramic thermal barrier coating occurs via conduction and radiation. Whereas the conduction of heat through these materials via phonon transport remains quite low over a wide range of temperature, the translucent nature of ceramic materials can allow for significant levels of heat transfer via radiation as the temperature increases. The heat transfer problems associated with thermal radiation are exacerbated in modern aircraft engines because of their high combustor pressures, which maximize the production of efficiently radiating carbon particulates, and their high peak combustion temperatures. Thermal radiation can contribute as much or even more to overall heat transfer than convective processes in these engines, particularly as temperatures increase.
Unlike metallic materials which are opaque, the translucent nature of oxide ceramics allows for direct heat transfer via radiation over certain wavelengths. The amount of heat transferred through the ceramic via radiation during service at high temperature depends upon the predominant wavelengths of the incident radiation, the optical properties, such as emissivity and absorption coefficient, of the coating material, and the coating thickness. The optical transmittance of a thermal barrier coating comprised of yttria-stabilized zirconia is such that 80% of incident radiation in the 1-3 μm wavelength range is transmitted through a 0.002″ coating. Since radiation emitted by the combustion gases, which contain water and carbon dioxide, will be concentrated in this wavelength range for the temperatures typically encountered during service, reducing radiation heat transport through the ceramic coating will enhance the insulating properties of the thermal barrier coating.